This invention relates to an optical isolator device transmitting an incoming light beam which is propagated through the optical isolator device along a forward direction and cutting off a return light beam which is propagated through the optical isolator device along a backward direction opposite to the forward direction.
As well known in the art, an optical isolator device is for use in an optical fiber transmission system which comprises a light source, such as a semiconductor laser device and an optical fiber. The optical isolator device is disposed between the light source and the optical fiber. The light source emits a light beam which is incident as an incoming light beam onto an incident end of the optical isolator device. The optical isolator device transmits the incoming light beam which is propagated through the optical isolator device along a forward direction. From an outgoing end of the optical isolator device, the incoming light beam is incident as an outgoing light beam onto an incident end of the optical fiber. A part of the outgoing light beam is reflected on the incident end of the optical fiber as a reflected light beam. Therefore, the reflected light beam is returned as a return light beam to the outgoing end of the optical isolator device. The return light beam is propagated through the optical isolator device along a backward direction opposite to the forward direction. The return light beam is cut off by the optical isolator device to prevent the light source from receiving the return light beam. As a result, the light source operates with stability and the optical fiber transmission system operates at a high optical transmission quality.
A known optical isolator device comprises first Faraday rotator which is disposed between the first and the second polarizers. The first and the second polarizers are arranged along a common optical axis and have first and second planes of polarization which are inclined at 45.degree. to each other.
The incoming light beam, which is propagated along the forward direction, is incident on the first polarizer of the optical isolator device. The first polarizer polarizes the incoming light beam into a linearly polarized light beam with a plane of polarization which coincides with the first plane of polarization. The linearly polarized light beam is sent to the magneto-optical element. The magneto-optical element rotates the plane of polarization of the linearly polarized light beam to produce a rotated light beam with a plane of polarization which is inclined at 45.degree. to the first plane of polarization and which coincides with the second plane of polarization. The rotated light beam passes through the second polarizer as an outgoing light beam from the optical isolator device. A part of the outgoing light beam is reflected as the reflected light beam on an incoming/outgoing end of an external optical element, such as an optical fiber.
The reflected light beam is returned to the second polarizer as a returned light beam. The returned light beam is propagated along the backward direction opposite to the forward direction. The returned light beam is incident on the second polarizer. The second polarizer polarizes the returned light beam into another linearly polarized light beam with a plane of polarization which coincides with the second plane of polarization. The linearly polarized light beam is sent to the magneto-optical element. The magneto-optical element rotates the plane of polarization of the linearly polarized light beam to produce another rotated light beam with a plane of polarization which is inclined at 45.degree. to the second plane of polarization and which is orthogonal to the first plane of polarization. Therefore, the rotated light beam does not pass through the first polarizer, whereby the first polarizer cuts off propagation of the rotated light beam.
The above-described optical isolator device, which comprises the pair of the polarizers and the magneto-optical element, is called a single optical isolator. This is because the above-described optical isolator device is a minimum unit which acts as the optical isolator device. It will be noted here throughout the instant specification that the single optical isolator may be referred to as an optical isolator element. As well known in the art, the optical isolator device is generally characterized by an isolation characteristic which represents a difference between a ratio of intensity of an incoming light beam to intensity of an outgoing light beam in a case of passing the light beam of a predetermined intensity through the optical isolator device in the forward direction as the incoming light beam and another ratio of intensity of an incoming light beam to intensity of an outgoing light beam in another case of passing the light beam of the predetermined intensity through the optical isolator device in the backward direction as the incoming light beam. The single optical isolator normally has an isolation characteristic of -30 dB.
However, this isolation characteristic is insufficient in a case where it is necessary for the semiconductor laser device to operate at a high stability for an oscillation frequency or the like. In order to obtain a higher isolation characteristic, an optical isolator device of a high-isolation type has been proposed, for example, in Japanese Unexamined Patent Prepublication of Kokai No. Sho62-189,422, namely, 189,422/1987. The proposed optical isolator device comprises two optical isolator elements which are arranged in series along a common axis, thereby obtaining an higher isolation characteristic greater than -60 dB.
A conventional optical isolator device of the high-isolation type, such as the proposed optical isolator device is, however, disadvantageous in that it is impossible to obtain a sufficient isolation characteristic in a wavelength range apart from an oscillation wavelength of a laser light beam when the optical isolator device is for use in cutting off a return light beam which is sent to the semiconductor laser device. This is because each polarizer in the optical isolator device is a polarizing beam splitter which has a relatively narrow polarizer cutoff wavelength band. Polarizers having a relatively wider polarizer cutoff wavelength bands, such as polarizers made of, for example, calcite, rutile, or the like are extremely expensive.